diff --git a/Communication.md b/Communication.md
index d218771..05b52c8 100644
--- a/Communication.md
+++ b/Communication.md
@@ -36,6 +36,7 @@ Commands from _execution thread_ to _listening thread_:
 - **done** - notifying of finished job. Requires 2 message frames:
     - `job_id` - identifier of finished job
     - `result` - response result, one of "OK" and "ERR"
+    - `message` - non-empty error description if result was "ERR"
 
 
 ### Progress callback